A. Field of Invention
The subject invention pertains to pacemakers, and more particularly to a pacemaker with means for stabilizing the ventricular rate, during atrial tachyrhythmia/fibrillation, by pacing the ventricle at successively higher rates until a steady state condition is established for the ventricle.
B. Description of the Invention
One problem experienced by some heart patients is a random depolarization or beat in the atrium, usually referred to as atrial fibrillation. Atrial fibrillation, while uncomfortable, does not constitute an immediate danger, by itself, to the patient and accordingly, the normal clinical approach to such a condition is to ignore it or to treat it in either an inadequate or a drastic manner. However, one indirect result of atrial fibrillation is an irregular ventricular rate. In other words, during atrial fibrillation, the ventricular intervals may vary substantially from one ventricular event to the next. The mechanism for this biological phenomenon is not understood completely. It has been suggested that, during atrial fibrillation, the AV node receives numerous successive stimuli originating from the atrium, and while each stimulus alone has a low amplitude which is insufficient to trigger a ventricular contraction, they do cause partial depolarizations. The effects of these partial depolarizations is cumulative, so that when a sufficient number of such stimuli are received, the AV node is depolarized resulting in unstable random ventricular contractions. (See R. J. Cohen et al, QUANTITATIVE MODEL FOR VENTRICULAR RESPONSE DURING ATRIAL FIBRILLATION, IEEE Transactions on Biomedical Engineering Volume 30, pages 769-782 (1983)). Ventricular instability is undesirable because it is uncomfortable for the patient. Presently, symptomatic patients are treated with drugs which are frequently ineffectual and/or have undesirable side effects or they are treated with AV nodal/ablation, a drastic procedure. However, stabilizing the ventricular rate during atrial fibrillation can lead to improved cardiac output, diastolic blood pressure, pulmonary artery pressure and end diastolic mitral valvular gradient. (C-P Lau, Leung, C-K Wong, Y-T Tai, C-H Cheng. A NEW PACING METHOD FOR RAPID REGULARIZATION OF RATE CONTROL IN ATRIAL FIBRILLATION, Am J Cardiol 65:1198-1203, (1990)).
It has been found that during atrial fibrillation, the ventricle can be stabilized at a rate approximately equal to the average intrinsic ventricular rate. Again, the mechanism of how the ventricle is stabilized by a pacing rate lower than the maximal intrinsic ventricular rate is not completely understood. It has been suggested that ventricular pacing eliminates the spontaneous depolarization phase of the AV node. F. H. M. Wittkampf, M. J. L. DeJongste, RATE STABILIZATION BY RIGHT VENTRICULAR PACING IN PATIENTS WITH ATRIAL FIBRILLATION. PACE 9:1147- 1153 (1986). F. H. M. Wittkampf, M. J. L. DeJongste, H. I. Lie, F. L. Meigler. EFFECT OF RIGHT VENTRICULAR PACING ON VENTRICULAR RHYTHM DURING ATRIAL FIBRILLATION, J Am Coll Cardiol 11:539-545, (1988). These articles disclose that the ventricle may be stabilized by using 93-97% of the sensed and paced ventricular depolarizations for pacing. However this method would pace atrial flutter and atrial fibrillation with a regular ventricular response which would not be beneficial and result in overpacing. The proposed method provides a negative feedback (closed-loop control) on pacing and stability, whereas the previous method was open loop.
Another procedure (see Lau, supra) that was investigated to stabilize ventricular instability during atrial fibrillation was to apply an additional stimulus at a preselected interval after every sensed conducted ventricular beat. The average interval was about 230 ms. However it is believed that this procedure is unsatisfactory because it may result in proarrhythmia by pacing the ventricle during the vulnerable period of ventricular repolarization.